The Castle School is a coeducational secondary school and sixth form with academy status, located in Thornbury, South Gloucestershire, England, which serves the town and the surrounding villages. Pupils from Bristol also attend the school. There are around 1,189 pupils, including 360 in the sixth form.

Thornbury is a market town and civil parish in the South Gloucestershire unitary authority area of England, about 12 mi (19 km) north of Bristol. It had a population of 12,063 at the 2011 census. The population has risen to 14,496 in the 2021 census. Thornbury is a Britain in Bloom award-winning town, with its own competition: Thornbury in Bloom. The earliest documentary evidence of a village at "Thornbyrig" dates from the end of the 9th century. The Domesday Book of 1086 noted a manor of "Turneberie" belonging to William the Conqueror’s consort, Matilda of Flanders, with 104 residents.